1. Field of the Invention
The present invention relates to a novel copolymer and use thereof. More particularly, the present invention relates to a copolymer including a constituent unit derived from ethylene and a constituent unit derived from an α-olefin and having specific properties, a lubricating oil viscosity modifier including the copolymer, and a lubricating oil composition including the lubricating oil viscosity modifier.
2. Description of Related Art
Petroleum products generally have so-called “a temperature dependence of viscosity”, i.e., the viscosity increases as the temperature decreases, and decreases as the temperature increases. For example, it is preferred that a lubricating oil used for automobiles and the like have a small temperature dependence of viscosity. Therefore, a specific polymer soluble in a lubricating oil base stock is used for a lubricating oil as a viscosity modifier in order to reduce the temperature dependence of viscosity. In recent years, an ethylene/α-olefin copolymer has been widely used as such a viscosity modifier. In order to provide a lubricating oil with a further improved performance balance, various improvements have been made on the viscosity modifier.
A viscosity modifier as described above is generally used to suppress a decrease in viscosity of a lubricating oil at a high temperature. In recent years, a viscosity modifier has been demanded which also suppresses an increase in viscosity of a lubricating oil at a low temperature (i.e., excellent low-temperature properties). A lubricating oil composition is used in applications in which a shear stress is applied. Therefore, a lubricating oil composition is required to exhibit excellent shear stability. A polymer having a low molecular weight is generally used to improve shear stability. On the other hand, it is necessary to increase the amount of viscosity modifier added along with a decrease in molecular weight in order to obtain viscosity required as a lubricating oil, whereby economic efficiency deteriorates. Moreover, there is a problem that reducing the amount of viscosity modifier added deteriorates shear stability by using a viscosity modifier having a high molecular weight to improve the economic efficiency of a lubricating oil.
Various catalysts have been studied in order to narrow the molecular weight distribution of a polymer, and a catalyst supported on highly active magnesium chloride, a metallocene catalyst, a vanadium catalyst and the like have been proposed. A polymer produced using such a catalyst having an Mw/Mn of 1.2 is known (see JP-A-S60-35009, for example).
It is known that appropriate low-temperature properties are obtained when using a blend of ethylene/α-olefin copolymers differing in the amount of constituent unit derived from ethylene as a viscosity modifier for a lubricating oil. However, the resulting lubricating oil exhibits an insufficient performance balance (see U.S. Pat. No. 3,697,429, for example). Another innovation has been adopted utilizing the properties of living polymerization. For example, a random copolymer and a block copolymer of ethylene and an α-olefin, which have a narrow molecular weight distribution and a narrow composition distribution and the composition of which changes in the molecule, have been disclosed. These copolymers have excellent shear stability, thickening properties, and low-temperature properties particularly suitable for lubricating oil applications, and are known to be suitable as a lubricating oil viscosity index improver. In particular, it is preferable to use a block copolymer having segments with different compositions in one molecule (see JP-A-S60-35009, for example). Use of a tubular reactor has been disclosed as a method of continuously obtaining a polymer having a narrow molecular weight distribution using a living polymerization catalyst. When producing a copolymer using a tubular reactor, the monomer composition in the reactor changes with the passage of time since the monomers differ in reactivity, whereby a tapered copolymer is obtained which has a wide intramolecular composition distribution in which the composition changes stepwise in the polymer chain. The polymer disclosed in the JP-A-60-35009 is suitable as a viscosity index improver for a lubricating oil. However, an improvement in low-temperature properties is insufficient when using the polymer disclosed in the JP-A-60-35009 having a narrow molecular weight distribution with an Mw/Mn of about 1.2, and further improvement is required. As a method of improving low-temperature properties, a method is known which increases the amount of constituent units derived from ethylene in the polymer while avoiding a range in which the low-temperature properties deteriorate to a large extent. However, since the intramolecular composition distribution of a tapered copolymer is wide, it is difficult to avoid the range.
A lubricating oil viscosity modifier formed of an olefin block copolymer has been proposed (see JP-T-2002-507225, for example). One of the blocks of the olefin block copolymer disclosed in the JP-T-2002-507225 contains about 93 wt % of constituent units derived from ethylene, whereby the olefin block copolymer exhibits an insufficient improvement effect of low-temperature property.
As another olefin block copolymer used for a lubricating oil viscosity modifier, an olefin block copolymer is known which contains an ethylene/α-olefin polymer block containing a large amount of constituent units derived from ethylene and an ethylene/α-olefin polymer block containing a large amount of constituent units derived from an α-olefin (see JP-A-2003-48931, for example).
Lubricating oil viscosity modifiers having other configurations are also known (see WO00/60032, JP-A-2003-48931, and JP-B-6-96624, for example).
However, a lubricating oil composition using the related-art lubricating oil viscosity modifier must be improved in terms of low-temperature fluidity.